The invention relates to injection devices for injecting set doses from an ampoule mounted in the device, which doses are set by operation of a dose setting button by which operation an elastic member is deformed to store an amount of energy which can be released to inject the set dose.
Syringes are known in which a helical spring is compressed to store energy sufficient both for automatic insertion of a needle into the skin of a user and for injection of a set dose through the inserted needle. As the force delivered by such a spring is proportional with the compression and the first part of the expansion when the spring is released is used for insertion of the needle whereas the later part of the expansion is used for the injection of the set dose from the ampoule, the characteristic of the spring must be chosen as a compromise which allow both functions.
In U.S. Pat. No. 5,320,609 separate coil springs are used for the two purposes so that each spring can be chose with the characteristic which is best for its purpose. A dose setting mechanism is so designed that setting of a dose results in a compression of the injection spring which compression can be released to perform the injection. A sequencing mechanism takes care that the injection is not made until the needle has been inserted into the skin of the user by decompression of the needle insertion spring.
Apparatuses are known which uses the same helical spring for both the needle insertion and the injection of medicine but benefits from the fact that a coil spring has different characteristics depending on the way it is used either as a source of an axial force provided by axial compression of the spring or as a source of a torque provided by rotating one end of the coil relative to the other about the longitudinal axis of the coil.
In both the above mentioned devices the characteristics of the injection springs are such that the spring force increases proportionally with the deformation so that the injection pressure will be initially be high and then fade out during the injection.
In U.S. Pat. No. 5,478,316 is described a device in which a constant force spring is provided for exertion of the injection force on the piston. The spring has the shape of a rolled up strip and the spring force is provided as the strip seeks to roll up on the roll again.
All the springs mentioned changes their dimensions during use as one part of the spring is moved relative to the other. By coil springs the ends of the coil is pressed towards each other and space must be made for the expansion of the spring. When the ends of the coil spring are rotated relative to each other the diameter of the coil changes and space must be made to allow such changes. By the constant force spring described the spring strip roll is moved relative to the end drawn off the roll and space must be made for this movement. The syringe housing must encompass all the space mentioned and this way said space contributes to an enlargement of the dimensions of the syringe. As slim syringes are aimed at, it may be seen as a way to enable minimising of a syringe if the space needed for the function of an injection spring is minimised.
According to the invention an injection device as described in the opening of this application is characterised in that the elastic member is a torsion rod which is twisted by the operation of the dose setting member.
The torsion rod may be placed parallel with the longitudinal axis of the syringe with its distal end fixed relative to the syringe housing, and the toque may be transmitted from the dose setting button to the proximal end of the rod by a gear transmission.
The gear transmission may appropriately be established by a toothing carried by the dose setting member to rotate with this member, which toothing engage a gear wheel fixed to the proximal end of the torsion rod. The toothing carried by the dose setting member may appropriately have a larger diameter than has the gear wheel at the proximal end of the rod.